The present invention relates to thermal clutches and more particularly to an improved thermal clutch with a very short time constant.
It takes time for an object suddenly immersed in a new thermal environment to reach the temperature of the new environment. The process is quantified by a measure called the thermal time constant. The thermal time constant equals the time it takes for the object to reach 63.2 per cent of the difference between the old temperature and the new temperature. For a very thin ( 1/32nd of an inch diameter) piece of high conductivity copper wire, suddenly immersed in still air at a different temperature, the thermal time constant is a surprisingly long-one minute (60 seconds).
However, there is a need for a thermal clutch (an object much more massive than the thin piece of copper wire) with a thermal time constant of less than a second. A fast-reacting thermal clutch would improve an automatic controlled-descent device (U.S. patent application Ser. No. 10/757,956—Apparatus for Exterior Evacuation from Buildings) used for escaping from the upper floors of a burning building. The thermal clutch's function would be to rapidly de-couple the device's descent-slowing mechanism when the temperature of the surrounding medium goes to or above a predetermined temperature such as 200° F., and then to rapidly re-couple the decent slowing mechanism again when the temperature of the surrounding medium goes below 200° F. The thermal clutch requires a low (<1 second) thermal time constant in order to cause the person wearing the descent device to quickly accelerate (indeed, free-fall) through an intense hot zone resulting from a lower fire floor in less than a second, and then to quickly decelerate on the other side of the intense hot zone. By passing through the 200+° F. temperatures so fast, the person avoids getting burned—much as a circus tiger avoids getting burned when rapidly leaping through a flaming hoop, and one avoids being burned if one passes his finger through a 1,450° F. flame in a quarter of a second. Air temperatures below 200° F. can be endured for longer periods, as confirmed by the many people who spend time at those temperatures in saunas every day.